As to the phenomenon of oxidation, I personally feel its importance is overstated. (I'm not entirely sure exactly what time frame you're referring to as 'too long', either.) As long as the whisky is stored properly (both before and after opening), I'd say you don't have a great deal to worry about until the contents are significantly diminished. And if you're really worried about any potential degeneration, you can always transfer the remaining contents to a smaller bottle and reseal. I do this quite frequently, and it works like a charm.
As for any possibility of degeneration in the bottle, I'd guess this might (and I emphasize 'might') be an issue more with whiskies that display very delicate aromatic qualities or with whiskies that possess pronounced, rather singular characteristics (such as smokiness).
What IS truly an issue is EVAPORATION! But if you promptly recap any whisky bottle after pouring, this won't be a problem either.
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